This proposal seeks to study how the chick eye adjusts its growth to compensate for defocusing spectacle lenses that impose myopia or hyperopia. This lens-compensation is seen as a model both for emmetropization--the process by which the eye adjusts its refractive state during development-and for how certain visual environments might predispose young human eyes towards myopia. Because brief, frequent episodes of lens-wear cause surprisingly good compensation, but extremely brief episodes are less effective, we propose to study the temporal integration of lens-wearing episodes by the emmetropization control mechanism. Furthermore, because episodes of wearing plus lenses block myopia from negative lenses, we propose to study this interaction by switching lenses over a wide range of frequencies. We also propose to examine chemical signals in the retina and choroid to see if the time course of their response can explain this enduring effect of plus lenses. The chemical signals we propose to study are retinoic acid and glucagon, both of which show opposite changes in levels to plus and minus lenses, suggestive of their involvement in the control eye growth. Finally, we propose to study possible visual signals that the eye might use to discern myopia from hyperopia. By these means we hope to understand the mechanism by which very brief episodes of wearing plus lenses can cancel out an entire day of wearing minus lenses. Such an understanding would strengthen the rationale for arresting myopic progression in children by visual manipulations.